Gain-controllable amplifiers are important particularly in mobile telephones, where the majority of the power is consumed in generating the output power for the transmitter. The output power must be adjustable in order to reduce power consumption when the connection to the base station can be maintained with a power level lower than the full output power. For instance in a GSM system, the base station controls in a centralized manner the output power of all telephones located within its range, in order to minimize their power consumption. Typically the output power of a transmitter is controlled by adjusting the amplification of the power amplifier of the transmitter.
Normally, a cellular telephone operates at less than the maximum transmitting power. The full power is needed in a cellular network only near the edges of the cells, i.e. when the distance to a base station is long. The cellular network controls the transmitting power of the mobile units by measuring the received strength of the signals transmitted by the mobile units and transmit commands to the mobile units to adjust their transmitting power to a suitable level. If the received signal is too weak, the particular mobile unit is given a command to raise its transmitting power. On the other hand, if the strength of the received signal is so high, that the transmitting power can be decreased without essentially decreasing the signal quality, the mobile unit is given a command to reduce its transmitting power. This kind of transmitting power control is used to optimize the power consumption of the mobile units, thereby lengthening the time a mobile unit can be used without recharging its battery. Also, the reduction of transmitting power also reduces the amount of interference the transmission of different mobile units cause to each others' signals. The power levels used by typical digital cellular telephones, such as GSM, PCN, or DCS cellular telephones, can typically be varied in the range 0 dBm to +33 dBm (1 mW to 2 W). This level range is so large, that the characteristics of the power amplifier stage, for example linearity, are very different at the minimum power level and at the maximum power level.
In high frequency technology, there are known several ways for controlling the amplification of an amplifier, such as controllable transconductance, controllable feedback, current partitioning and the use of a controllable attenuation stage. All these control methods are described in more detail in the specification below.
The amplification of the amplifier stage can be adjusted for instance by changing the transconductance of the amplifier transistor. A general feature of transistors is the dependence of the transconductance g.sub.m on the quiescent current of the transistor, and hence with a suitable adjustable amplifier transistor bias circuit an adjustable amplifier can be created. As an example of this type of an amplifier stage, FIG. 1 illustrates a differential emitter-coupled pair 2, the transconductance g.sub.m whereof depends on the level of the current I.sub.E. The current I.sub.E is controlled by a constant current element 1. This type of fully differential circuits have the drawback that their linear range of operation is limited. However, structures based on a differential pair, are well suited to be integrated, wherefore they are generally used in devices realized with integrated circuits.
Amplification can also be controlled by adjusting the feedback resistance of the amplifier stage. FIG. 2 illustrates a simple basic circuit arrangement of an amplifier stage, where the amplification of the amplifier stage can be controlled by means of the resistance Rf. An adjustable resistance is often accomplished by using a PIN diode or a FET transistor. In microwave technology, this type of structure is often realised with discrete components in a hybrid circuit. Some drawbacks of this structure are the limited scale of bandwidth and amplification range.
An amplification control circuit applying current partitioning adjusts the current to be directed past the load resistance. In the exemplary circuit of FIG. 1, the current steered by the differential pair 2 passes as a whole via the load resistances R.sub.L. When a Gilbert quad is added to this type of circuit, in a manner illustrated in FIG. 3, part of the current drained by the differential pair can be conducted past the resistances R.sub.L, in which case the circuit amplification decreases in proportion to the currents passing via the resistances R.sub.L and past them. A drawback of this type of control circuit is poor efficiency, because the current conducted past the load resistance is not utilized.
An adjustable amplification can also be accomplished by means of an attenuator to be coupled as a preliminary stage for the amplifier stage. In an arrangement of this type, the amplification of the actual output stage is constant, and the amplification of the whole amplifier coupling is controlled by increasing or decreasing the attenuation of the attenuator. This type of attenuator can be realised for instance by using PIN diodes.
As a conclusion of the prior art arrangements, let us point out that most solutions have drawbacks with respect to efficiency, linearity or dynamic range. Moreover, many of the known solutions are difficult to accomplish with current microcircuit techniques, and are therefore poorly suited for instance to portable radio devices, where the aim is to use compact components that take up as little space as possible.
Problems with linearity of a certain signal processing block such as an amplifier can be corrected to a large extent by adding a predistorter stage whose linearity characteristics are the inverse of the signal processing block. Such solutions are exemplified by the system described in the European patent application EP 720112, where a predistorter stage counters the errors introduced by nonlinearities in a signal processing block. One other example is the Finnish patent FI 98014, which describes the use of a predistorter stage prior to an amplifier stage for correcting the nonlinearities of the amplifier stage. These solutions have the drawback that by adding a functional block, they increase the complexity of the system and the manufacturing costs.